The requirement to reduce pollutant emissions has motivated turbine manufacturers to develop advanced combustion technologies. Although capable of producing ultra-low emissions (<10 ppm NOx), these advanced combustors suffer from flame instability problems such as flashback, combustion dynamics, and lean blowout. These problems cause reduced component life, unplanned shutdowns, and potentially catastrophic engine damage. Flame instabilities can be triggered by weather changes, fuel composition changes, operational changes, and component wear. To avoid these costly problems, turbine manufacturers have typically developed operating margins at the expense of ultra-low emissions. An alternative strategy is to perform in-situ combustion monitoring to provide the feedback necessary to minimize pollutant emissions while avoiding combustion instabilities. A combustion control and diagnostics sensor (CCADS) for gas turbines is the subject of U.S. Pat. Nos. 6,429,020; 6,887,069; and 7,096,722.
The CCADS flame ionization sensor 10 is based on two electrically isolated electrodes installed on the fuel nozzle as shown in FIG. 1. The electrode closest to the combustion zone is called the guard electrode 12, and the upstream electrode is called the sense electrode 14. When an equal voltage is applied to both electrodes, this arrangement facilitates current flow between the guard electrode 12 through the flame in the combustion region. As a result, the guard electrode signal can provide a wealth of important information about flame stability and the combustion process. A significant ionization current from the sense electrode 14 is produced only when the flame enters the upstream region of the fuel nozzle, i.e., during auto-ignition and/or flashback. The multi-sensing capability of CCADS flame ionization sensor 10 provides a simple, yet robust, in-situ monitoring sensor for combustion diagnostics.
However, quantifying important operating parameters for control of the turbine, e.g., equivalence ratio control, over the entire load range is complicated by flame instabilities. For example, during dynamic pressure oscillations at the peak pressure the flow through the system slows allowing the reaction to sometimes enter the premixing region of the fuel nozzle. The resulting dynamic changes in flame location complicate the CCADS measurement for equivalence ratio. Significant changes to the combustion conditions, such as those required for a large load change, i.e., change in bulk flow velocity, can result in flame variations that also affect the correlation of the CCADS measurements. In modern Dry Low NOx (DLN) gas turbines these types of changes are common while operating over the entire load range. To effectively implement CCADS for control of gas turbine combustors, an improved method for quantifying CCADS measurements is necessary.
Current CCADS measurements provided by the three aforementioned patents are achieved using a direct current (DC) measurement technique. A DC voltage is applied to the sensor electrodes resulting in a steady electric field projected into the combustion region, and the measured current through the flame is analyzed for combustion diagnostics. The extension of that invention provided by the present approach is to use advanced measurement techniques to mitigate the affects of flame instabilities as described in detail below. In accordance with an embodiment of the invention, numerous combinations of time-varying voltage (AC) and DC voltage can be applied to the sensor electrodes to generate a time-variant electric field projected into the combustion region. These advanced measurements provide additional information about flame electrical properties that can be used to improve sensor capability to accurately determine quantifiable measurements for combustion control applications.